When is a weapon mastered?

Im trying to level up my assault rifles, im Ego 309 but only assault rifle 4. Is a rifle considered 'mastered' once the bar that is between the ammo count and XP bar has been filled once, or when the xp bar in the inventory, the one that is something like 654012/885000 has been filled?

That's your weapon xp, once that is full you won't earn anymore Assault Rifle proficiency. You'll need to equip a new AR and level that up. Your AR proficiency is 5 tabs right and that has a huge bar, you'll probably be level 1500 and just using AR's before that's full

So the XP bar that is in the menu that has the 654012/885000 is weapon XP but the bar between the two is the class XP

The class xp for weapons is on the stats page, it'll say AR level 4 and then what bonus you have and whats coming up.

the one that says 600000/800000 is the weapon xp, true.